In the spark-discharge surface-treatment technique, spark discharge is effected between an electrode and a metallic surface to be treated as they are brought into and/or out of mutual contact, with a brief electrical impulse applied between them which is of an intensity sufficient to produce localized heating of the relatively small discharge-impinging area. By sweeping such contact discharge over a selected surface region of the workpiece, a metallurgical modification or hardening of this selected surface area is obtained. Using these principles, the coating of a metallic workpiece with a metal or alloy which is different from the substrate, for example, carbide coating, can be achieved with a firm metallurgical bond between the substrate surface and the coated layer.
As shown in Japanese Patent Specification No. 32-9998 issued Nov. 29, 1957, for example a precoat layer of coating material may be applied to a workpiece surface to be treated and an electrode in the form of a solid rotary member may be moved or rolled over the precoat while urging it against the surface as an electric impulse is repeatedly applied between the electrode and the workpiece to fuse the precoat to the receiving workpiece surface at successive locations. Even without such a precoat, however, the electrode may itself form a source of coating material. An improved system and practical applications may thus result which uses the fusion-transfer of a material to a workpiece surface from the electrode in a rotary disk or similar form in sliding or tangential movement over the surface with the aid of repeated contact discharges. Such systems have likewise been well and long known in the art as described, for example, in Japanese Patent Specification No. 32-599 issued Jan. 29, 1959, No. 32-2446 issued Apr. 19, 1959, No. 32-2900 issued May 16, 1959 and No. 32-6848 issued Aug. 28, 1959. In these methods, the material fusion-transfer contact discharge can be repetitively effected by a capacitor circuit designed to charge and instantaneously discharge across the points of contact between the electrode and the workpiece and recharge as the contact region shifts from one contact to the next on points between the electrode and the workpiece. Otherwise, a mechanical or electrical switching of a continuous voltage source has been employed to provide periodically a pulsed voltage across the moving interface of the electrode and the workpiece.
In a method shown in U.S. Pat. No. 3,098,150 issued July 16, 1963, an electrode chip is repeatedly driven into contact with a workpiece, for example, under a spring force applied to the electrode held resiliently upon an electrode holder. A spark discharge is drawn between the tip and the workpiece from a charged capacitor, thereby creating a partial weld between them. Coupled with the electrode holder, there is an electromagnetic coil designed to be energized at least in part by the charging current of the capacitor or a short-circuit condition between the electrode and the workpiece. The coil is thus operable, upon the capacitor discharge, to draw the electrode tip abruptly away from the workpiece in order to break the weld and leave material from the electrode tip deposited upon the workpiece. Of course, the coating material may be disposed in advance between the electrode and the workpiece, here again, independently of the electrode material.
According to the aforementioned electrode vibration method, each metal fusion and deposit cycle is sharply controlled by the electrode reciprocation with each stroke of the cycle advantageously synchronized with capacitor discharge and recharge, thus permitting more consistent and uniform deposition than with the other prior system utilizing a rotary electrode in which contact discharges are produced randomly over the continuous contact region of the electrode and the workpiece in a continuous or intermittent displacement.
A significant disadvantage of this method has now been found to lie in the use of a capacitor, especially in conjunction with the use of vibration cycles. The vibration must be synchronized with the capacitor charge and recharge cycle, and hence requires the relative long period of each mechanical cycle. Consequently, there is a severe limitation in the frequency of discharge impulses and hence in the rate of deposition attainable. Another restriction is found also in the lack of flexibility to alter the operating parameters over a wide range as desired, which is required where a variety of electrode and workpiece materials and finishing requirements are to be met.
In summary, it may be said in practical terms that conventional spark deposition or surface treatment methods, regardless of whether they are of the vibrating or rotary type, are undesirably limited to achieve satisfactory results as regards the rate of deposition in their ability or treatment, the consistency of deposition, the stability of operation and the uniformity of the deposited surface, especially where an improvement in one is desirable without sacrifice of the others.